To help put the invention into context, we will first briefly describe some salient points of the Global Positioning System (GPS).
The GPS comprises a set of orbiting satellites. The GPS has several modes of operation, e.g. the L1 mode and the L2 mode. In each of these modes, each satellite repeatedly transmits a navigation message. For the purposes of this document, the signal formed by the repeated transmission of a satellite's navigation message will be referred to as the satellite's payload signal. A GPS receiver is designed to receive these navigation messages and to use navigation messages from different satellites in combination in order to estimate the GPS receiver's position. The GPS satellites modulate their payload signals on to radio frequency carrier signals that are then transmitted for reception by GPS receivers.
In each operating mode of the GPS, the satellites all use the same radio frequency carrier signal, although different operating modes of the GPS do however use different radio frequency carrier signals. Therefore, GPS must provide a way for a GPS receiver to distinguish between payload signals that belong to the same GPS operating mode but which arrive from different satellites.
This is achieved by providing each satellite with a different Gold code and arranging that each satellite spreads its payload signal with its unique Gold code (unique, that is, within the set of GPS satellites) before modulating the payload signal on to a radio frequency carrier signal. Thus, a GPS receiver needs to despread a received payload signal with the correct Gold code and that allows a GPS receiver to not only distinguish between payload signals from different satellites but also to identify from which satellites the payload signals have been sent. Effectively then, GPS uses a form of code division multiple access (CDMA) in which Gold codes are used for the spreading/despreading.